3,874 research outputs found
Observable Dirac-type singularities in Berry's phase and the monopole
The physical reality and observability of 2n\pi Berry phases, as opposed to
the usually considered modulo 2\pi topological phases is demonstrated with the
help of computer simulation of a model adiabatic evolution whose parameters are
varied along a closed loop in the parameter space. Using the analogy of Berry's
phase with the Dirac monopole, it is concluded that an interferometer loop
taken around a magnetic monopole of strength n/2 yields an observable 2n\pi
phase shift, where n is an integer. An experiment to observe the effect is
proposed.Comment: 12 pages Latex, 3 postscript figures; submitted to Physical Review
Letters 15 September 2000; revised 19 November 200
Topological properties of Berry's phase
By using a second quantized formulation of level crossing, which does not
assume adiabatic approximation, a convenient formula for geometric terms
including off-diagonal terms is derived. The analysis of geometric phases is
reduced to a simple diagonalization of the Hamiltonian in the present
formulation. If one diagonalizes the geometric terms in the infinitesimal
neighborhood of level crossing, the geometric phases become trivial for any
finite time interval . The topological interpretation of Berry's phase such
as the topological proof of phase-change rule thus fails in the practical
Born-Oppenheimer approximation, where a large but finite ratio of two time
scales is involved.Comment: 9 pages. A new reference was added, and the abstract and the
presentation in the body of the paper have been expanded and made more
precis
Tackling 3D ToF Artifacts Through Learning and the FLAT Dataset
Scene motion, multiple reflections, and sensor noise introduce artifacts in
the depth reconstruction performed by time-of-flight cameras. We propose a
two-stage, deep-learning approach to address all of these sources of artifacts
simultaneously. We also introduce FLAT, a synthetic dataset of 2000 ToF
measurements that capture all of these nonidealities, and allows to simulate
different camera hardware. Using the Kinect 2 camera as a baseline, we show
improved reconstruction errors over state-of-the-art methods, on both simulated
and real data.Comment: ECCV 201
Relationship of Decan volcanism to global K-T biotic extinctions: a case of unfounded assumptions?
This article does not have an abstract
The Geometric Phase and Ray Space Isometries
We study the behaviour of the geometric phase under isometries of the ray
space. This leads to a better understanding of a theorem first proved by
Wigner: isometries of the ray space can always be realised as projections of
unitary or anti-unitary transformations on the Hilbert space. We suggest that
the construction involved in Wigner's proof is best viewed as an use of the
Pancharatnam connection to ``lift'' a ray space isometry to the Hilbert space.Comment: 17 pages, Latex file, no figures, To appear in Pramana J. Phy
Iron meteorite fall at Bhuka village, Barmer District, Rajasthan
This article does not have an abstract
Modular group algebras with almost maximal Lie nilpotency indices. I
Let K be a field of positive characteristic p and KG the group algebra of a
group G. It is known that, if KG is Lie nilpotent, then its upper (or lower)
Lie nilpotency index is at most |G'|+1, where |G'| is the order of the
commutator subgroup. The authors have previously determined the groups G for
which this index is maximal and here they determine the G for which it is
`almost maximal', that is the next highest possible value, namely |G'|-p+2
The first interferometric detections of Fast Radio Bursts
We present the first interferometric detections of Fast Radio Bursts (FRBs),
an enigmatic new class of astrophysical transient. In a 180-day survey of the
Southern sky we discovered 3 FRBs at 843 MHz with the UTMOST array, as part of
commissioning science during a major ongoing upgrade. The wide field of view of
UTMOST ( deg) is well suited to FRB searches. The primary beam
is covered by 352 partially overlapping fan-beams, each of which is searched
for FRBs in real time with pulse widths in the range 0.655 to 42 ms, and
dispersion measures 2000 pc cm. Detections of FRBs with the UTMOST
array places a lower limit on their distances of km (limit of
the telescope near-field) supporting the case for an astronomical origin.
Repeating FRBs at UTMOST or an FRB detected simultaneously with the Parkes
radio telescope and UTMOST, would allow a few arcsec localisation, thereby
providing an excellent means of identifying FRB host galaxies, if present. Up
to 100 hours of follow-up for each FRB has been carried out with the UTMOST,
with no repeating bursts seen. From the detected position, we present 3
error ellipses of 15 arcsec x 8.4 deg on the sky for the point of origin for
the FRBs. We estimate an all-sky FRB rate at 843 MHz above a fluence of 11 Jy ms of events sky d at the 95
percent confidence level. The measured rate of FRBs at 843 MHz is of order two
times higher than we had expected, scaling from the FRB rate at the Parkes
radio telescope, assuming that FRBs have a flat spectral index and a uniform
distribution in Euclidean space. We examine how this can be explained by FRBs
having a steeper spectral index and/or a flatter log-log
distribution than expected for a Euclidean Universe.Comment: 13 pages, 8 figures, 2 table
The UTMOST pulsar timing programme I: overview and first results
We present an overview and the first results from a large-scale pulsar timing
programme that is part of the UTMOST project at the refurbished Molonglo
Observatory Synthesis Radio Telescope (MOST) near Canberra, Australia. We
currently observe more than 400 mainly bright southern radio pulsars with up to
daily cadences. For 205 (8 in binaries, 4 millisecond pulsars) we publish
updated timing models, together with their flux densities, flux density
variability, and pulse widths at 843 MHz, derived from observations spanning
between 1.4 and 3 yr. In comparison with the ATNF pulsar catalogue, we improve
the precision of the rotational and astrometric parameters for 123 pulsars, for
47 by at least an order of magnitude. The time spans between our measurements
and those in the literature are up to 48 yr, which allows us to investigate
their long-term spin-down history and to estimate proper motions for 60
pulsars, of which 24 are newly determined and most are major improvements. The
results are consistent with interferometric measurements from the literature. A
model with two Gaussian components centred at 139 and fits the transverse velocity distribution best. The pulse duty
cycle distributions at 50 and 10 per cent maximum are best described by
log-normal distributions with medians of 2.3 and 4.4 per cent, respectively. We
discuss two pulsars that exhibit spin-down rate changes and drifting subpulses.
Finally, we describe the autonomous observing system and the dynamic scheduler
that has increased the observing efficiency by a factor of 2-3 in comparison
with static scheduling.Comment: 31 pages, 14 figures, 6 tables, accepted for publication in MNRA
Geometric Phases and Multiple Degeneracies in Harmonic Resonators
In a recent experiment Lauber et al. have deformed cyclically a microwave
resonator and have measured the adiabatic normal-mode wavefunctions for each
shape along the path of deformation. The nontrivial observed cyclic phases
around a 3-fold degeneracy were accounted for by Manolopoulos and Child within
an approximate theory. However, open-path geometrical phases disagree with
experiment. By solving exactly the problem, we find unsuspected extra
degeneracies around the multiple one that account for the measured phase
changes throughout the path. It turns out that proliferation of additional
degeneracies around a multiple one is a common feature of quantum mechanics.Comment: 4 pages, 4 figures. Accepted in Phys. Rev. Let
- …